In a code division multiple access (hereinafter referred to as “CDMA”) communication system, after modulation of a signal to be sent, the signal is multiplied by a diffusion code to perform spectral diffusion, followed by sending the multiplied signal to a destination. On the destination side, before demodulation, the received signal is again multiplied by a code, to perform reverse diffusion to reproduce the signal sent from the send side. The diffusion code used on the destination side is the same diffusion code as used on the send side but is opposite in sign.
When the CDMA communication system is adopted, the allocation of individual diffusion codes to respective users can realize communication with each of the users.
Efficient utilization of multi-path is one characteristic feature of the CDMA communication system. In the case of radio communication, in addition to the case where the signal sent from the transmitter as such is straightly received by the receiver, there is a situation where the signal is diffracted or attenuated by objects which block off the signal, and a phenomenon called “fading” takes place upon the movement of the transmitter. When the diffraction takes place, a signal, which has passed through a plurality of paths, is received by the receiver. A signal, which has caused phase difference or time difference at the receipt time, is called “multi-path,” and each received signal in the multi-path is called “path.”
In TDMA (time division multiple access) and FDMA (frequency division multiple access) as other communication systems for sending a plurality of signals, when one signal in the multi-path is a main signal as a signal to be received, the other path signals are handled as noise which obstructs the main signal. In the CDMA communication system, however, the multi-path can be separated into individual paths all of which are handled as the main signal.
FIG. 1 illustrates “rake receive” as a method for this purpose (rake received is discussed below). In this example, a receive signal 11 is input into each of first to third reverse diffusion circuits 121 to 123. These first to third reverse diffusion circuits 121 to 123 extract respective path signal components 131 to 133. A synthesis circuit 14 uses these signal components 131 to 133 as respective main signals and regulates the output timing of these components so as to match the output timing of one signal. As a result, an output signal 15, corresponding to the sum of the three signal components 131 to 133, is obtained from the synthesis circuit 14.
This technique, wherein the signal sent from the send side using the same diffusion code is collected in this way as if the signal is raked to enhance the receive sensitivity, is called “rake receive.” Further, a circuit, which functions to detect the path of the received signal using the same diffusion code and to separate the path, is called a “searcher.” Japanese Patent Laid-Open No. 4211/2000 discloses a circuit for the rake receive technique.
FIG. 2 is a schematic diagram showing the construction of a searcher circuit in a conventional communication apparatus. A searcher 21 comprises: a delay unit 24 which permits the input of a received signal 23 obtained from a receive end 22 and delays the received signal 23; a correlator 25 for examining the correlation of waveforms; an averaging section 26 for averaging delay profile representing the relationship between each receive timing of the receive signals and the correlation value; a path detector 27 for detecting a path; a delay control unit 28 for controlling the level of delay of the delay unit 24; and a finger section 29. In this searcher 21, a value of correlation with a pilot signal as a known signal contained in the received signal 23 is computed by means of the correlator 25 while shifting the delay level in the delay unit 24 by means of the delay control unit 28.
FIG. 3 shows an example of a change in delay profile upon the movement of a mobile unit. As a mobile unit such as a portable telephone (a cellular telephone) moves from FIG. 3A toward FIG. 3B and FIG. 3C in that order with the elapse of time, the relationship between the receive timing plotted as abscissa and the correlation value plotted as ordinate changes. In these drawings, arrows 31a, 31b, and 31c indicate a search range as the receive timing range for computing the correlation value.
Thus, the delay profile changes with the elapse of time upon a change in the position of the mobile unit. Therefore, the averaging section 26 shown in FIG. 2 averages a plurality of acquired delay profiles for enhancing the reliability of the path detection in the path detector 27. In fact, even in receive timing in which a path is absent, the correlation value becomes in some cases high as a probability event due to some cause. When this result as such is input into the path detector 27, a path is detected. This is causative of erroneous detection. The path detector 27 searches for receive timing having a high average correlation value in the delay profiles, and detects this as a path which is then notified to the finger section 29.
The cycle, at which path detection is carried out in the path detector 27, depends upon the number of delay profiles which are averaged by the averaging section 26. Specifically, when the number of delay profiles to be averaged is increased from the viewpoint of enhancing the reliability of the path detection, the cycle for the path detection is long. In the case of mobile communication, there is a possibility that the mobile unit moves with the elapse of time. As the mobile unit moves, the search range 31 shown in FIG. 3 changes.
On the other hand, the range which the correlator 25 can compute the correlation value at a time is limited by the restriction of hardware as the communication apparatus. In general, the correlation value computation range is several times the delay spread as the spread of the delay profile. Therefore, the searcher 21 cannot search at a time the whole range in which the mobile unit can exist. For this reason, the searcher 21 controls the delay control unit 28 so that, as indicated by arrows 31a, 31b, 31c shown in FIG. 3, the search range in terms of time range for computing the correlation value is moved according to the location at which the path exists at that time.
In the searcher 21 of the conventional communication apparatus, as described above, the path detection cycle cannot be satisfactorily shortened. This is because shortening the cycle has a fear that the reliability of the delay profile after the averaging in the averaging section 26 is lowered and erroneous path detection takes place. Therefore, disadvantageously, this cannot cope with ever-changing path position. Of course, the hardware can be improved so as to overcome this problem. This, however, poses problems of an increase in size of hardware and a significant increase in cost.